1. Field of the Invention
The present invention relates to an image density control method and a color image forming apparatus such as a color copying machine. More particularly, the present invention relates to an apparatus which develops latent images formed for different colors by using corresponding color toners under predetermined conditions, and superposes the resultant different color toner images to form a single color image, and to an image density control method performed by the apparatus.
2. Description of the Related Art
A color image forming device such as a color reproduction machine usually includes a latent image carrying member such as a photosensitive drum on which latent images for the respective colors of an image to be finally formed are formed and respective developing devices which are disposed close to the latent image carrying member and use corresponding color toners. The latent images for corresponding colors are developed by the corresponding color developing devices under predetermined conditions to be formed into corresponding color toner images, and these color toner images are finally superposed to form a color image.
There are two known methods of controlling the respective density of the different color images in a color image forming device of this type:
(1) One method is to sense the surface potential of the latent image carrying member using a surface potential meter, to correct the surface potential to a constant value and to maintain it at that constant potential (Published Unexamined Japanese Patent Application (Kokai) Nos. 57-100,448, 57-163,241, 57-163,242 and 58-217,960), and
(2) The other method is to deposit a toner on the surface of a latent image carrying member or on a developing capability sensor provided in the developing device, to irradiate light onto the deposited toner layer, to sense the reflection to determine the developing capability and to control the toner density such that the developing capability becomes constant (Published Unexamined Japanese Patent Application (Kokai) Nos. 60-73,655 and Published Unexamined Japanese Utility Model Application (Jikkai) No. 55-162,253).
However, according to the above methods, finally, only the density of a high density portion of a color image can be controlled and it is impossible to cope with a possible loss of the color balance in the medium or low density portion of the color image where color changes are most likely to appear.
Solely by correcting and maintaining the surface potential at a constant value, it is impossible to cope with fluctuations of the developing capability due to the supply of an excessive quantity of toner. As a result, the toner density may abnormally increase thereby to create smears in the low image density portions (high light portions).
Especially the copy density versus original document density characteristics of toner colors are different from each other and fluctuate depending on the state of toners used and on environmental fluctuations. Furthermore, developing capabilities such as the quantity of electric toner charges also fluctuate depending on aging and environmental fluctuations. Therefore, it is difficult to obtain a stabilized color balance.